The present invention relates to respiratory therapy apparatus and more particularly relates to respiratory therapy apparatus having improved means for entering set values of various parameters and displaying the actual measured values of operational variables.
Respiratory therapy apparatus such as ventilators, anesthesia machines and other devices for controlling the flow of respiratory gases to and from a subject such as a medical patient may be arranged to regulate a plurality of different operational variables. Thus, a typical medical ventilator may be arranged to regulate the volume inhaled and exhaled by the patient on each breath, referred to as the "tidal volume", the breath rate or number of breaths per unit time and other variables. The ventilator typically incorporates automatic devices for monitoring each of these variables to provide measured values and controlling the apparatus based upon stored set values for various parameters. The parameters may correspond directly to the controlled variables. Thus, a stored set value of tidal volume may be used for control of the actual tidal volume used in operation of the apparatus. Alternately, a parameter may correspond indirectly to one or more operational variables. Thus, a stored set value of a parameter referred to as "inspiratory flow rate" may be used in control of variables related to the inspiratory flow rate but not having a direct, 1:1 correlation thereto. The stored set values for the parameters may include upper or lower limits for each parameter or, more typically, a so-called center point or desired value for each parameter.
The set values typically have been entered into the apparatus by actuation of movable setting elements such as a rotary knob for each parameter. Typically, each such knob has been provided with a conventional pointer and scale or other mechanical indicator directly connected to the knob. Thus, the pointer or other mechanical indicator shows the set value for the associated parameter. Typically, the measured values for the variables have been displayed on gauges, digital read outs and the like linked to the control system. The measured values thus have been displayed separately from the set values shown by mechanical indicators associated with the knobs.
These arrangements are less than optimal in many respects. It is generally difficult to obtain an accurate reading of the set value from the mechanical indicator associated with the knob. Factors such as parallax between a mechanical pointer and scale and the limited resolution available in a mechanical scale of practical size limit the accuracy with which the physician can discern the set value. Moreover, the correlation of the position of the knob and the set value entered into the apparatus is not perfect. Typically, the control system employed in the apparatus is electronic, and the knob or other movable setting element is mechanically linked to a variable electronic element such as a potentiometer. Imperfections in the mechanical linkage between the knob and the electronic element, such as shifting of a knob on the shaft of the potentiometer may alter the correlation between position of the knob and the value of the variable electronic component. Thus, imperfections in the mechanical linkage may alter the correlation between the value indicated by the mechanical pointer or indicator associated with the knob and the actual set value entered into the electronic system. Changes in the characteristic of the variable electrical element, such as changes in the resistance characteristics of a potentiometer, may have a similar effect. For all of these reasons, the mechanical indicators associated with the setting elements generally do not provide accurate indications of the set values which have been entered into the control apparatus and stored therein. Accordingly, it is difficult for the physician to detect subtle deviations of the measured values from the values expected in view of the stored set values.
Additionally, this system is inconvenient for the physician. Comparison of set and actual values may require the physician to look first at the knob and associated mechanical indicator or scale to ascertain the set value, and then at a remote display to ascertain the actual value. Where the physician desires to adjust a set value and observe the change in the actual measured values, he must continually shift his eyes back and forth from the actual value display to the mechanical indicator showing the set value. These problems are particularly serious inasmuch as respiratory therapy apparatus may be used in critical care situations where the physician must maintain close, careful surveillance of the respiratory therapy and also perform other tasks necessary to patient care. Thus, there have been significant unmet needs heretofore for improvements in respiratory therapy apparatus.